1. Field of the Invention
The present invention relates generally to a current control circuit and more particularly to a current control circuit comprising field-effect transistors capable of controlling currents even if the power source potential fluctuates sharply.
2. Description of the Related Art
It is known that field-effect transistor bias voltage supply circuits, as shown in FIGS. 16, 18, have been used to determine currents in the art. In the bias voltage supply circuits shown in FIGS. 16, 18, the current-voltage characteristics between the source and drain of a field-effect transistor having the gate and drain connected together are utilized. The characteristics of such circuits deviate from the constant voltage characteristics with an increase in the current. In other words, from the transistor characteristics it can be said for bias voltage generator circuits shown in FIGS. 16, 18 that a transistor having the drain and the gate connected together has the characteristic that the drain-source voltage increases with an increase in drain current based on the drain-source voltage characteristics of a saturation region in a case that the gate and drain of the transistor are connected. The bias circuits of FIGS. 16, 18 are applied to conventional operational amplifiers 3A, 3A illustrated in FIGS. 17, 19. The conventional operational amplifiers 3A, 3A illustrated in FIGS. 17 and 19 are the same as that of which details are mentioned below with reference to FIG. 4. The bias voltages at terminals 20, 20xe2x80x2 of FIGS. 16 and 18 are supplied to gates of P-channel field-effect transistors MP14 and MP15 setting currents flowing in the operational amplifiers 3A, 3A of FIGS. 17 and 19, wherein the transistors MP14 and MP15 supply and set the current to a differential input portion comprising transistors MP12, MP13, MN8 and MN9 and an output portion comprising a transistor MN10, respectively. The, circuit currents i10 and i510 of the operational amplifiers 3A, 3A depend on the respective power source voltages VDDs as i10 of FIG. 6 and i510 of FIG. 7, and increase with an increase in the power source voltage. This presents the problem that the performances of the operational amplifiers shown in FIGS. 17, 19 according to the related art can be assured only in a narrow range of power source voltages. More specifically, when the conventional voltage generator circuit for providing constant current in combination with a transistor with a realistic channel width and channel length is used as a constant current source for an amplifier, the circuit current increases with an increase in the power source voltage, resulting in large fluctuations of the frequency response and stability margin. This causes the problem that the operational range of the power source potential is narrowed.
In addition, such conventional constant current circuits are disclosed in Japanese Patent Laid-Open No. 42717/1989 and Japanese Patent Laid-Open No. 91166/1993. The constant current circuits disclosed in these unexamined publications have had the problem that it is more difficult to obtain the constant current characteristics in low voltage range of twice to several times the voltage of the threshold Vth of a transistor to be used compared to a higher power source voltage range.
According to one aspect of the invention, it is an object of the invention to provide a current control circuit capable of maintaining constant current characteristics with respect to a wide range of power source potential fluctuations.
According to another aspect of the invention, it is another object of the invention to provide a current control circuit for use in an amplifier of which a stable performance is required even if large fluctuations of a power source voltage in use arise in the circuit wherein a battery is used as an electric source depending on the degree of consumption of the battery.
Furthermore, in the conventional circuit, the fluctuation property of current shows monotonous straight-line form when constant current circuits are connected in the multi-stage form, so that it is impossible to provide the characteristic that power source voltage fluctuations are canceled out and the combined characteristic that circuit current goes up first and later down with respect to the source voltage. Therefore, circuit current has not been maintained within a given range regardless of the source voltage fluctuations with the use of the combined characteristic.
According to a still further aspect of the invention, it is an object of the invention to provide a current control circuit having a nonlinear output characteristic with respect to power source voltage fluctuations.
In addition, according to even further aspect of the invention, it is an object of the invention to provide a current control circuit with desired characteristics and large design freedom so that it can be constituted by combining transistors each having the realistic channel width and channel length.
To solve the problems and achieve the objects, according to the invention, there is provided a current control circuit comprising: a first resistor with one end connected to a power source potential; first and second P-channel field-effect transistors, each of which having a source connected to the other end of the first resistor and a gate coupled to a gate of the other P-channel field-effect transistor, the first P-channel field-effect transistor having a drain directly connected to both the gates coupled together; a second resistor through which a drain of the second P-channel field-effect transistor is connected to both the gates coupled together; and a resistor element through which both the gates coupled together are connected to a zero potential, wherein a voltage arising at the drain of the second P-channel field-effect transistor is used as a gate-driving voltage for driving a gate of a current-setting transistor.
The current control circuit may be used in combination with their one or more equivalents. In addition, the resistor element may comprise transistors constituting a current mirror circuit for duplicating a current flowing through a current-setting transistor which receives a driving voltage from one of the current control circuits used for a pre-stage.
Furthermore, according to the invention, there is provided a current control circuit comprising: a first resistor with one end connected to a zero potential; first and second N-channel field-effect transistors, each of which having a source connected to the other end of the first resistor and a gate coupled to a gate of the other N-channel field-effect transistor, the first N-channel field-effect transistor having a drain directly connected to both the gates coupled together; a second resistor through which a drain of the second N-channel field-effect transistor is connected to both the gates coupled together; and a resistor element through which both the gates coupled together are connected to a power source potential, wherein a voltage arising at the drain of the second N-channel field-effect transistor is used as a gate-driving voltage for driving a gate of a current-setting transistor.
The current control circuit may be used in combination with their one or more equivalents. In addition, the resistor element may comprise transistors constituting a current mirror circuit for duplicating a current flowing through a current-setting transistor which receives a driving voltage from one of the current control circuits used for a pre-stage.
The current control circuits according to the invention can maintain current consumption of the circuit constant even if the operating power source potential range is widened, so that it is useful for amplifiers. In other words, it is possible to provide a current control circuit capable of maintaining a fixed performance over a wide range of the power source potential.
A combination of the current control circuits according to the invention has the characteristic that the current consumption goes up first and later down with an increase in the power source potential instead of the linear characteristic that the current consumption monotonously increases with an increase in the power source potential, whereby better control of current consumption can be provided than conventional current control circuits.
In addition, according to the invention, there is provided a current control circuit in combination with a current control circuit of the opposite conductivity type. A resistor element thereof may include the current-setting transistor, which receives the driving voltage from the pre-stage current control circuit.
Therefore, with the current control circuit according to the invention, it is possible to provide a current control circuit for maintaining constant current characteristics with respect to a wide range of power source potential fluctuations. Further, according to the invention, it is also possible to provide an amplifier capable of achieving stable performance even if large fluctuations of a power source voltage in use arise in the circuit wherein a battery is used as an electric source depending on the degree of consumption of the battery. Also, according to the invention, it is possible to provide a current control circuit having the fluctuation characteristic that circuit current goes up first and later down with an increase in the source potential, and the nonlinear output characteristic with respect to the power source voltage fluctuations, whereby a current fluctuation range can be reduced. In addition, according to the invention, it is possible to provide a current control circuit with desired characteristics and large design freedom so that it can be constituted by combining transistors each having the realistic channel width and channel length.